Light intensity comparison with other render engines

[rawnsley]

I'm aware this is a can of worms and I'm not expecting pixel-perfect matching, but I'm struggling with the most basic comparison. I expect I'm doing something wrong, but I can't see it.

Sadly there are no definitive examples, but the KHR_lights_punctual extension PR has a simple one that appears good enough to start with: KhronosGroup/glTF#1223 (comment)

The initial renders don't seem to be correct, but eventually a consensus is reached from a mix of tools KhronosGroup/glTF#1223 (comment). Here is the output in glTF Viewer:

If I load the same file into Filament's gltf_viewer the plane is completely black (once sunlight is disabled).

I have to boost the light intensity by a factor of a thousand to get something similar to the other render engines.

If anything Filament is giving the more believable result. The "default" light in the scene has an intensity of 1 candela and I would not expect a single candle held half a metre above a plane to have much visible effect and certainly not reach the ground nearly four meters away.

[romainguy]

The problem is that glTF does not define the exposure of the camera. The other renderers use a very sensitive exposure by default equivalent to f/1 1s ISO 100 in Filament. We just had a call with Khronos about this and I'm going to write a proposal for an extension that adds exposure parameters to glTF cameras. To match other renderers you can use Camera::setExposure(1) in Filament.